Abstract

Analysis of the 2phi -3phi heavy-mineral fraction of samples from a transect of the continental shelf off Georgia reveals that hornblende constitutes more than 25% of the non-opaque, non-micaceous heavy-mineral suite in three areas of the shelf; the shelf edge, the central shelf, and the near shore area. Data from previous work by Giles and Pilkey (1965) indicate that Georgia rivers can not be the source of these hornblende-rich sands. The Santee River of South Carolina is a possible source of hornblende-rich sands, and longshore drift is the most probable mechanism of transport to the shelf off Georgia. Holocene beach-dune sands south of the Santee River are composed of hornblende-rich sands from northern sources diluted with more mature sands reworked from the Silver Bluff terrace. Hornblende-rich sands in the central-shelf and shelf-edge areas are composed principally of sands from northern sources carried along the shelf by longshore currents during periods of stillstand at sea levels lower than present sea level, in Pleistocene, probably Wisconsin, time. Low-hornblende sands of the inner and outer shelf were derived primarily from local sources during regression or transgression and are mineralogically similar to Georgia river sands. Sediment sources and transport mechanisms on the Georgia coast vary with sea level oscillation. During regression major sources of sand-sized sediment are the Piedmont and coastal plain of Georgia and the nearshore sea bottom. During transgression sediment from landward sources is trapped in estuaries and becomes bottom sediment with continued transgression. Sediment eroded from headlands and reworked beach-dune and estuarine sediment are the major sources of shore-face sands during transgression. Longshore drift operates during both transgression and regression, but residence times are long and the net drift rate low. During stillstand the supply of sediment from the adjacent mainland is reduced, longshore-drift rates are high, and hornblende-rich sand from northern sources is the dominant sediment

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